• It makes a scene: I, personally, don't like attention to be drawn to myself when I'm in a classroom surrounded by people I don't like or know.  I'm sure there's a billion people that enjoy such things, but I'm sure not one of them.
• Stating the obvious: No one knows how long they were gone better than the person who was gone.  Telling them so makes me believe you are trying out to be the lead in a McDonald's comercial.  Say something constructive besides "You're back!  You've been gone a long time!"  Unless, of course, you're attempting to make it on someone's "Top Ten Annoying Morons" list.
• Repeat Factor: Chances are, the prodigal son is going to hear the question "Where were you?!" at least twenty times in a seven-hour period.  Show some courtesy: shut your yap.
• Disbelief: If you do ask the above question and actually get an answer that doesn't involve your face being shoved into a brick wall or other such painful maneuvers, please keep in mind: the person who was absent is probably pretty sure of why they were gone.  Therefore, telling them they're lying only shows how shallow your brain matter is.  Not only that, it's very annoying.  If they are lying, it's their right and thinking they owe you the absolute truth is a sign of the advanced stages of brain loss (curable only by means of a sharp object inserted into your skull).

Now, I'm sure that they're your friend and they love you and are happy and all that jazz to see you, so it's good to somehow acknowledge their absence.  However, being a jumpy, yappy moron about it isn't a good way to invoke feelings of comaraderie.